City targeted for the first thousand-plane air raid in World War II. Although the British had first bombed German industry in 1940, their successes until 1942 had been minimal. The British strategic bombing campaign suffered from a lack of bombers (prior to 1942, the Royal Air Force [RAF] Bomber Command never possessed more than 400 bombers) and an inability to hit targets with any accuracy at night. In the spring of 1942, the war was going badly for the Allies, and the Soviet Union was pressing for a second front. As the only alternative for direct offensive action against Germany, the British changed their policies on strategic bombing.

With the accession of Air Chief Marshal Sir Arthur Harris as commander of Bomber Command in February 1942, the British began a deliberate program of targeting built-up areas instead of industries. In a desperate need to boost morale at home and to demonstrate the increasing capabilities of Bomber Command, Harris proposed to Prime Minister Winston L. S. Churchill the idea of a thousand-plane raid. The plan immediately received Churchill's approval. By temporarily stripping Training Command of aircraft and securing other planes from Coastal Command, Harris assembled 1,086 aircraft, more than enough planes to cover any aborted takeoffs. The Admiralty then withdrew the 250 Coastal Command aircraft, but Harris was determined to reach the thousand-plane figure, even though it meant sending some crews who were not yet fully trained.

Köln (Cologne) and Hamburg were the two possible targets. They were selected over the more valuable Essen because each was located on a large body of water and could be readily identified by Gee (for G or grid), a new navigational aid. On the day of the raid, bad weather ruled out Hamburg, but Köln was clear. Known as Operation millennium, the raid occurred on the night of 30–31 May 1942. As it worked out, Harris got 1,046 bombers aloft, 600 of them Wellingtons. His goal was to pass all aircraft over Köln in only 90 minutes.

The air raid on Köln, Germany's fifth-largest city, lasted approximately 100 minutes, with the British bombers passing over the city at an average of 11 per minute. A total of 890 bombers reached Köln and dropped more than 1,455 tons of bombs on the city, two-thirds of them incendiaries. The bombing resulted in the destruction of an estimated 13,000 homes and the razing of almost 600 acres of the city. Casualties amounted to 469 dead and over 5,000 wounded on the German side, with another 45,000 left homeless. The British lost over 300 crewmen in the 41 bombers that failed to return, a loss of just 3.8 percent. The large number of bombers overwhelmed the German fighter defenses. The considerable destruction of this raid captured the imagination of the British public amid a series of Allied defeats in North Africa and East Asia.

Following the Köln raid, the British also launched thousand-plane raids against Essen and Bremen in June 1942. The Köln raid confirmed to Harris and Bomber Command the viability and effectiveness of night area bombing. The raid also shattered German illusions about who was winning the war and caused Adolf Hitler to lose confidence in the Luftwaffe. But instead of accepting the need to strengthen Germany's air defenses, Hitler ordered retaliatory German air attacks.